How tamoxifen works
Tamoxifen is a so-called selective estrogen receptor modulator (SERM). This means that its estrogen-inhibiting effect is cell- and tissue-specific.
Tamoxifen inhibits the effect of oestrogen in breast tissue (antagonistic) while it has an agonistic effect in the uterus, bones or lipid metabolism.
The endogenous female hormone oestrogen (also known as estrogen) not only determines a woman’s cycle, but also has other tasks in the body. Among other things, it ensures strong bones (a lack of oestrogen can lead to osteoporosis) and stimulates the immune system.
When oestrogens are released in the body, they reach the target tissue via the bloodstream. Once there, they specifically influence the target cell and can stimulate cell growth, among other things.
If a cell has many docking sites (receptors) for oestrogens, it reacts particularly sensitively to the hormone. An increased number of oestrogen receptors is found in a large proportion of breast tumors.
The already degenerated cells are further stimulated to grow and divide, i.e. multiply, by the natural oestrogen, causing the tumor to grow uncontrollably.
Absorption, breakdown and excretion
After ingestion, the active substance is well absorbed in the intestine and reaches its maximum blood level after four to seven hours. Metabolism, which mainly takes place in the liver, leads to degradation products that are many times more effective.
These are then excreted mainly in the stool, but this takes some time. It takes about a week for half of the active substance to be broken down and excreted.
When is tamoxifen used?
The active ingredient tamoxifen is approved for the treatment of hormone-dependent breast tumors. It can be used as a supportive treatment after primary treatment of breast cancer or for breast cancer that has already formed metastases.
It is usually used over a longer period of time. For example, if tamoxifen is used adjuvantly (to reduce the risk of recurrence), it is usually taken for five to ten years.
How tamoxifen is used
The active ingredient is administered in the form of tablets. The usual tamoxifen dosage is twenty milligrams per day, but can be increased to up to forty milligrams if necessary. It is taken with a meal to reduce undesirable effects such as nausea.
What are the side effects of tamoxifen?
One in a hundred to one in ten patients experience drowsiness, headaches, visual disturbances, vomiting, diarrhea, constipation, hair loss, hypersensitivity reactions, muscle pain, calf cramps, blood clots, temporary anemia and itching of the genitals.
Another side effect may be a change in laboratory values (increased blood lipid levels, altered liver enzyme values). As tamoxifen has an oestrogen-agonistic effect in the uterus, it can promote the rate of cell division there and thus the formation of polyps (mucosal growths) or carcinomas.
Have any unclear vaginal bleeding during tamoxifen therapy checked by a doctor immediately!
What should I bear in mind when taking Tamoxifen?
Contraindications
Tamoxifen must not be taken during pregnancy or while breastfeeding.
Interactions
Tamoxifen therapy is intended to reduce the effect of the body’s own oestrogen. An additional supply of oestrogen in the form of hormonal contraceptives (e.g. “the pill”) would not make sense and should therefore be avoided.
Tamoxifen influences blood clotting by reducing the number of platelets. If anticoagulant medication is also taken, the anticoagulant effect may be increased.
Tamoxifen is converted into the more active form by certain liver enzymes. Drugs that inhibit or promote the activity of these enzymes can thus influence the metabolism and therefore the effectiveness of the cancer drug.
For example, antidepressants from the group of selective serotonin reuptake inhibitors (SSRIs, such as paroxetine and fluoxetine) and the antidepressant bupropion can reduce the effectiveness of tamoxifen through enzyme inhibition. The simultaneous use of such drugs should therefore be avoided if possible.
Age restriction
Tamoxifen is not approved for children and adolescents under the age of 18.
Pregnancy and breastfeeding
As there is little data available on the use of tamoxifen during pregnancy and breastfeeding, the active substance must not be taken during this period. In animal studies, the use of tamoxifen caused damage to the unborn child.
How to obtain medication with tamoxifen
Preparations containing tamoxifen are available on prescription from pharmacies in Germany, Austria and Switzerland.
How long has tamoxifen been known?
As early as the late 1950s, pharmaceutical companies were actively researching anti-oestrogens (i.e. active substances that inhibit the effect of oestrogen) for effective contraception. Dr. Dora Richardson developed the active ingredient tamoxifen in 1966.
As a result, a clinical trial of tamoxifen was started in 1971 at the Christie Hospital in Manchester, one of the largest cancer clinics in Europe. Due to the positive study results, tamoxifen was marketed in 1973 for the treatment of late-stage breast cancer.
Other interesting facts about tamoxifen
Tamoxifen is abused by male athletes as a doping agent. It increases testosterone levels, which stimulates muscle growth. Tamoxifen also prevents a common side effect of anabolic steroids, the so-called “man boobs” (gynecomastia).
